These studies have focused on the regulatory properties of tyrosine hydroxylase, tryptophan hydroxylase and their cofactor tetrahydrobiopterin (BH4) in the biosynthesis and release of catecholamine and serotonin in the brain. We have identified two isoforms of human tryptophan hydroxylase on both mRNA and protein level. Human brainstem expresses only one of them while pineal gland has both. Preliminary data indicate that there are two promoters responsible for the two isoforms. These results provide clues for the analysis of the mechanism of regulation of tryptophan hydroxylase at the transcription level. By using surface plasmon resonance spectroscopy, we have demonstrated the binding between tryptophan hydroxylase that was phosphorylated by cAMP-dependent protein kinase and 14-3-3 proteins. 14-3-3 not only activates the tryptophan hydroxylase, but also protects it from dephosphorylation. Mutagenesis studies indicate that the major site in human brain tryptophan hydroxylase that is phosphorylated by cAMP-dependent protein kinase is serine-58 while serine-260, which is the site that is supposed to interact with 14-3-3, is only phosphorylated to a small extent. In addition, we have detected the formation of 4a-hydroxytetrahydrobiopterin during the hydroxylation of tryptophan by the recombinant tryptophan hydroxylase. The activity of tryptophan hydroxylase is increased by 2-fold in the presence of pterin 4a-carbinolamine dehydratase, the enzyme that converts the hydroxytetrahydropterin to the dihydropterin derivative.